How to speed up YOLOv3 detection speed?
Darknet is "native" framework, so basically, you don't need to implement anything, all code for yolov3 is available at their github repo, you just need to figure it out, play with it. Keras, in my opinion, is not flexible enough to easily implement yolo. If you want to implement yolo from scratch I would probably go with PyTorch it has a dynamic graph + more integration with python. Regarding speed, it's hard to say objectively which framework is faster, especially comparing with darknet, since probably nobody did a comparative analysis. The easiest solution to speed up yolo3 is obviously buying more powerful GPU :D.

Also, in my understanding what they did in yolov3 is that they intentionally sacrificed speed in order to be able to detect smaller objects, so if you don't care too much about small grouped up objects go with yolov2 it is very fast and has a pretty decent mAP.